1. Field of the Invention
The present invention relates to a Josephson junction device of an oxide superconductor having low noise level at liquid nitrogen temperature, and more specifically to a Josephson junction device of an oxide superconductor having a weak link of a Josephson junction consisting of a grain boundary between two single crystal oxide superconductor regions having low noise level at liquid nitrogen temperature.
2. Description of Related Art
A Josephson junction device is one of superconducting devices comprising two superconducting electrode connected through a Josephson junction. The Josephson junction device can be realized in various structures. Among the various structures, the most preferable structure in practice is a tunnel type Josephson junction device realized by a thin non-superconductor layer sandwiched between a pair of superconducting electrodes. However, a point contact type Josephson junction device, a micro bridge type Josephson junction device and a variable thickness bridge type Josephson junction device composed of a pair of superconductor electrodes which are weakly linked to each other also exhibit Josephson effect.
The Josephson junction device is expected to be applied to a high speed switching element and a high sensitive sensor due to its characteristics. In particular, by using high critical temperature oxide superconductors, recently advanced in study, for superconducting electrodes, it is possible to manufacture Josephson junction devices which operate under liquid nitrogen cooling.
These Josephson junction devices have fine structures so that fine processings are necessary to manufacture the Josephson junction devices. For example, in order to manufacture a tunnel type Josephson junction device, a first superconductor thin film, a non-superconductor thin film and a second superconductor thin film are stacked on a substrate in the named order.
The thickness of the non-superconductor layer of the tunnel type Josephson junction device is determined by the coherence length of the superconductor. In general, the thickness of the non-superconductor layer of the tunnel type Josephson junction device must be within a few times of the coherence length of the superconductor. In case of using an oxide superconductor for the superconducting electrodes, a thickness of a non-superconductor layer must be about a few nanometers, because oxide superconductor materials have very short coherence length.
However, the superconductor layers and the non-superconductor layer of the tunnel type Josephson junction device must be highly crystallized for favorable properties. It is difficult to stack an extremely thin and high crystalline non-superconductor layer on an oxide superconductor layer. Additionally, it is very difficult to stack a high crystalline oxide superconductor layer on the non-superconductor layer stacked on an oxide superconductor layer. Though the stacked structure including a first oxide superconductor layer, a non-superconductor layer and a second oxide superconductor layer is fabricated, the interfaces between the oxide superconductor layers and the non-superconductor layer are not in good condition so that the tunnel type Josephson junction device does not function in good order.
In the point contact type Josephson junction device, the micro bridge type Josephson junction device and the variable thickness bridge type Josephson junction device, two superconducting electrodes are contact with each other through an extremely small contact area so as to realize a weak link of a Josephson junction. Therefore, very fine processings are also necessary to manufacture the point contact type Josephson junction device, the micro bridge type Josephson junction device and the variable thickness bridge type Josephson junction device. It is very difficult to conduct the fine processings of oxide superconductors with good repeatability.
In order to resolve the above mentioned problems, so-called grain boundary weak link type Josephson junction devices are proposed in a prior art. A Josephson junction device of this type comprises a substrate having a principal surface and an oxide superconductor thin film formed on the principal surface of the substrate. The oxide superconductor thin film has two single crystal regions and a grain boundary between them, which form a weak link of a Josephson junction. Each of the single crystal regions of the oxide superconductor thin film constitutes a superconducting electrode. Thus, the above oxide superconductor thin film constitute a Josephson junction device.
The above grain boundary weak link type Josephson junction device can be manufacture by depositing an oxide superconductor thin film on a bicrystal substrate, a substrate having a proper seed layer on a portion of its principal surface or a substrate having a step on a portion of its principal surface.
No fine processing which is required to manufacture a point contact type Josephson junction device, a micro bridge type Josephson junction device or a variable thickness bridge type Josephson junction device is necessary to manufacture the grain boundary weak link type Josephson junction device.
In case of applying a Josephson junction device to a sensor, a problem of noise arises. When a Josephson junction device is used as a sensor, a power is supplied so that current flows through its Josephson junction and voltage changes between its two superconducting electrodes are measured.
However, there are noises in this voltage change and it can be easily understood that sensitivities of the sensors using Josephson junction devices can be improved by reducing noise level. In particular, the noises are noticeable in Josephson junction devices utilizing oxide superconductors at frequency lower than on the order of 100 Hz.
The noises at low frequency of the Josephson junction devices utilizing oxide superconductor thin films are primarily the sum of one caused by critical current fluctuations and one caused by flux motion and are dependent on 1/f (f: frequency). These are also temperature dependent so that their levels change as temperature of the Josephson junction devices change.
The critical current fluctuation noise becomes larger at a lower temperature and the flux noise has a peak near the critical temperature of the oxide superconductor. Hence, the noises of the Josephson junction devices utilizing oxide superconductor thin films become the minimum at temperatures little lower than the critical temperatures of the oxide superconductors.
H. K. Olsson et al. reported about the 1/f noise of dc SQUID utilized Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor in Appl. Phys. Letter vol. 61, No. 7, pp. 861-863, Aug. 17, 1992.
For example, the critical temperature of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor is higher than 85K and a temperature at which a Josephson junction device utilizing Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor has the minimum noise is on the order of 80K. Therefore, the Josephson junction device has a higher noise level at a liquid nitrogen temperature. In order to use the Josephson junction device at the temperature of the minimum noise, its temperature should be controlled by liquid helium cooling with heating by heater. Namely, the highest performance of the Josephson junction device can not be obtained under simple liquid nitrogen cooling.
The same is true for Josephson junction devices utilizing Bi--Sr--Ca--Cu--O type oxide superconductors and Tl--Ba--Ca--Cu--O type oxide superconductors. Since, the Bi--Sr--Ca--Cu--O type oxide superconductors and Tl--Ba--Ca--Cu--O type oxide superconductors have critical temperatures higher than 90K.